In the construction of oil and gas wells, it is occasionally necessary to cement a liner or casing in the well to provide stability and zonal isolation. In such processes, it is common to use plugs to separate different fluids pumped along the tubing or casing. Such plugs are usually installed in a basket located in cementing equipment lowered into the well. The plugs are launched from the basket by means of darts pumped from the surface.
A known cement plug launching tool (see U.S. Pat. No. 5,890,537) is shown in FIGS. 1-3. The body 32 of the launching tool includes an upper tubular housing 40 whose upper end is threaded to the mandrel of the liner setting tool, and whose lower end is threaded at 41 to a spacer tube 42. A sleeve valve 44 which is slidable in the bore of the housing 40 is biased upward to a normally open position with respect to ports 38 by a coil spring 46.
A piston 50 connected to a drive rod 36 slides in the bore of the spacer tube 42 which is connected to the upper end of a cylinder tube 55. A lower piston 58 is formed on the rod 36 and slides within the bore 60 of the cylinder tube 55 which is filled with a suitable hydraulic oil. The piston 58 has an outer diameter that provides a selected clearance with respect to the wall of the bore 60 such that, as the piston is forced downward with the rod 36, a metering effect is created which retards the rate of downward movement.
The lower end of the cylinder tube 55 is connected to the upper end of the basket 33 which initially houses the upper and lower wiper plugs 34, 35, and is provided with a plurality of longitudinal slots 68 that receive radial stop pins 70 which extend from the outer periphery of a drive flange 75 that rests on top of the upper plug 34. A head 71 on the upper end of the upper plug 34 receives the inner ends of several radially extending shear pins 73 on the drive flange 75 to releasably couple the plug 34 to the flange.
In operation and use, the liner is run and suspended by a hanger from a point near the lower end of the casing which is below the wellhead. The plug launcher tool is connected to the lower end of the mandrel, and the wiper plugs 34 and 35 were previously loaded into the basket 33. The drive rod 36 is in its upper position where the piston 58 is at the upper end of the oil chamber 60. The ports 38 in the housing 40 are open so that fluids can flow therethrough. A dart launcher is provided at the surface.
In order to cement the liner in place, cement slurry is pumped in through the dart launcher, and then a valve is opened to release a lower dart 101. Pressure is applied to the top of the dart 101 to force it through the valve and down into the drill pipe ahead of the cement. Eventually the dart 101 enters the housing 40, passes into the bore of the valve sleeve 44, and to a position where its nose bumps against the drive head 50 of the rod 36. Since the elastomer cups of the dart 101 seal off the bore of the valve sleeve 44, pressure causes the sleeve valve to shift downward against the bias of the coil spring 46, and in so doing, partially close off the radial ports 38.
Pressure on the dart 101 applies downward force to the rod 36 and causes it to shift downward in the body 32, thereby driving both the upper and lower wiper plugs 34 and 35 downward. Such movement is slowed by the action of hydraulic oil that meters upward through the clearance between the piston 58 and the inner wall of the cylinder 60 so that shock loads are dissipated. When the pins 70 on the drive plate 75 reach the bottoms of the slots 68 as shown in FIG. 2, downward movement of the upper plug 34 is stopped. However the lower plug 35 will have been ejected from the bottom of the basket 33 and into the bore of the liner. At about the same time as the stop pins 70 encounter the bottoms of the slots 68, the top cup of the dart 101 clears the bottom of the sleeve valve 44 so that the ports 38 are re-opened as the sleeve valve is shifted upward by the coil spring 46. Pumping of cement is continued until the desired number of barrels of cement has been placed within the liner.
When the proper amount of cement has been pumped into the running string, the upper dart 100 is forced into the drill pipe, followed by whatever fluid is being pumped behind it. The dart 100 travels down through the running string, the mandrel, and into the housing 40. When the cups of the dart 100 enter the valve sleeve 44 and seal off its bore, the valve sleeve shifts downward to close off the lateral ports 38. The dart 100 then engages the lower dart 101, so that applied pressures force the drive rod 36 further down in the body 32 as shown in FIG. 3. The pins 73 are sheared so that the drive disc 66 on the lower end of the rod 32 passes through the plate 75 and forces ejection of the upper wiper plug 34 from the bottom of the basket 33. The metering of oil past the piston 58 again slows or retards downward movement of the rod 32 so that ejection is smoothed. When the top end of the dart 100 clears the bore of the valve sleeve 44, the valve sleeve again opens, as before, so that displacement fluids flow around the outside of the launcher assembly and through the annular space between the basket 33 and the inner wall of the liner. A positive indication of the launching of wipers plugs 34 and 35 from basket 33 is shown by an increase in pumping pressure at the surface location resulting from the cushioned travel of piston 58 for both plugs 34 and 35. The shearing of pins 73 for upper plug 34 additionally increases the pumping pressure for upper plug 34. For example, the increase in the pumping pressure may amount to about 1500 psi for lower plug 35 and to about 3000 psi for upper plug 34.
As is discussed above, the plug launching system is activated by the launch of one or more darts. Darts are launched from modules that are operated by opening and closing a series of valves. Where more than one dart is launched the complexity of the dart launching equipment increases. Not only does this require more physical space but the process of opening and closing sets of valves makes the operation more complex and thereby less efficient. The increased complexity also means that the system is more prone to breaking down. In contrast ball dropping modules, which are commonly used to terminate operations, are more compact and much simpler mechanically. The major disadvantage of replacing darts with balls in such operations is that a ball is insufficient in length to provide the necessary stroke length to launch a plug.